454 tbi loping/surge

docstoy said:

So you never did anything with distributor pick up coil or distributor shaft ? When I said 20 miles I meant all at once and not 5 miles at a time. When I used to finish a repair on driveability it was at least a 30 minute test drive from in town (25 mph) to highway 55 to 75 mph , trying to induce all conditions a owner may put vehicle thru. Need to address misfire, correct that and then do the 20 miles and hopefully not else will happen, if it does address that issue and again after repair drive again, so on and so forth, nobody said driveability was easy and it isn't but is challenging . Just be patient you have already come a long way

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Distributor is brand new-mentioned in first post.

I get going for a longer drive. I can’t with how bad truck is missing. Just wanted to reference how far I did drive. Just looking for some ideas what else I can check?

Today I’m going to go check my scanner on my other truck, and I’m thinking of checking my grounds.

I did download service manual, if I’m understanding correctly the ecm is grounded at the water outlet? Or does it also use the firewall ground? I’m going to check them all.

Just left my storage. I have an 87 k10 that’s TBI so tried my scanner on that. It’s not showing the 2 values, just an *. Just like my dump truck. So must be something with my scanner not the truck

Long-term fuel trim is maxed-out but the O2 sensor is still saying it's lean at 1800 rpm.

TPS and MAP seem reasonable. It seems to clear up with more load so probably not fuel pressure.

I'd be looking for other sources of the trouble but it'd really suck if the fuel tables on that PROM were corrupted.

i do have a fuel pressure gauge installed at TBI on truck, i cant see it while driving, but its set at 12 right now at idle. i had it at 13 but turned it down a little just in case fuel pressure was causing an issue.

so just to clarify- when you say PROM, thats the "chip"? im 99% sure but just wanted to confirm

i did swap that- i had the fastchips "tune" when this all started, truck was loping bad at idle. i swapped in the stock prom last weds, truck now has this bad miss around 2k rpms.

could it be the O2 sensor? i have the one i took out 500 miles ago i could swap that in for a quick test. cost nothing but a few min of my time right now. this one seems to be working, but who knows?

could it be the actual ECM itself? do those fail in odd ways like this?

I did check grounds yesterday, this truck actually has upgraded grounds already at the rear engine- thick cable to firewall/frame/engine.

im just lost where to turn next. it seems ignition related- but distributor and wires are new, plugs look good. i checked coil with multimeter it tested good. could the ESC fail?

it seems to me, explaining in laymans terms here- for some reason the engine keeps thinking its running lean, and is adding fuel, when it doesnt appear it was, hence its running really rich

Tiger joe said:

i do have a fuel pressure gauge installed at TBI on truck, i cant see it while driving, but its set at 12 right now at idle. i had it at 13 but turned it down a little just in case fuel pressure was causing an issue.

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GM spec is 9--13. Should run just fine at 12--13, provided it stays at 12--13 under load; and the engine isn't modified.

Isn't the fuel pressure on the later TBI 454s supposed to be near 30? What year did that change? '93, I think. So 12--13 should be OK on your vehicle.

Tiger joe said:

when you say PROM, thats the "chip"?

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Yes.

Tiger joe said:

i swapped in the stock prom last weds, truck now has this bad miss around 2k rpms.

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I'm having some concern that the original PROM was faulty, which is why they bought and installed the hot-rod PROM.

But there's no knowing that for certain without reading the info on the PROM.

It's more-likely that there's some other problem causing the poor running.

Tiger joe said:

could it be the O2 sensor? i have the one i took out 500 miles ago i could swap that in for a quick test. cost nothing but a few min of my time right now. this one seems to be working, but who knows?

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Sure. It "could" be the O2 sensor. Or the sensor wiring.

Tiger joe said:

could it be the actual ECM itself? do those fail in odd ways like this?

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Anything is possible.

Tiger joe said:

im just lost where to turn next. it seems ignition related- but distributor and wires are new, plugs look good. i checked coil with multimeter it tested good.

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Will the ignition coil reliably fire a spark-tester calibrated for HEI, when the coil is fully-warmed and misted with water from a squirt-bottle?

Tiger joe said:

could the ESC fail?

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ANYTHING "can" fail.

Tiger joe said:

it seems to me, explaining in laymans terms here- for some reason the engine keeps thinking its running lean, and is adding fuel, when it doesnt appear it was, hence its running really rich

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How do you know that it's rich? The computer is in rich-command, but that doesn't mean the fuel delivery is actually rich. The O2 sensor is saying it's still lean.

just to clarify on the PROM- i installed the fastchips PROM myself years ago (maybe 5?) the stock PROM was working fine when removed.

when I started this thread with hot rod PROM- truck missing and loping horribly at idle. swap OEM PROM in, im 90% fixed, but now have the miss at ~2k.


i know the truck was running rich with old prom because it burned your eyes, choked you out in the garage with the raw fuel smell.

also i was always taught -backfire out intake- lean, backfire out exhaust- rich. so im using that logic that when i get this horrible miss driving down the road, and hear a backfire out the tailpipe, its rich.

i am trying not to just throw parts at the truck and do some actual diagnosis, but im struggling where to go from here.

when you say- spark tester calibrated for HEI- what exactly do you mean? one of the ones where you can adjust the gap to see if spark will jump X gap? i have one of those somewhere i need to dig out if so

TLR - I just read the few posts. Id like to clear up a couple things.

1.) After you changes the PROM back to the original did it run OK, then after some time it developed a miss fire?

2.) What brand distributor did you get? Just because it's new doesn't mean it's good. I had one that went bad before ~2000 miles.

3.) You said the tach started jumping all over the place. Was that while cruising at ~2000 RPM or around the same speed as when you're getting the backfiring?

The reason I ask these questions is, IMO they're all pointing towards a bad ICM and/or Ignition Coil. Here's a log I took on my 88 5.7L but, it should apply to your 454. Note the RPMs in the upper chart (black line), those 2 spikes turned out to be the ICM going bad. This is while cruising at ~2000 RPM - not when accelerating or decelerating. When it spiked the ECM thought it was running at that speed and adjusted fuel and spark accordingly causing a surge in the engine. Yours may be bad enough to cause a backfire.



Replacing the ICM cured the surge but it still stumbled a little at idle, changing the coil cured that (~2000 miles old too).

PlayingWithTBI said:

TLR - I just read the few posts. Id like to clear up a couple things.

1.) After you changes the PROM back to the original did it run OK, then after some time it developed a miss fire?

2.) What brand distributor did you get? Just because it's new doesn't mean it's good. I had one that went bad before ~2000 miles.

3.) You said the tach started jumping all over the place. Was that while cruising at ~2000 RPM or around the same speed as when you're getting the backfiring?

The reason I ask these questions is, IMO they're all pointing towards a bad ICM and/or Ignition Coil. Here's a log I took on my 88 5.7L but, it should apply to your 454. Note the RPMs in the upper chart (black line), those 2 spikes turned out to be the ICM going bad. This is while cruising at ~2000 RPM - not when accelerating or decelerating. When it spiked the ECM thought it was running at that speed and adjusted fuel and spark accordingly causing a surge in the engine. Yours may be bad enough to cause a backfire.

You must be registered for see images attach

Replacing the ICM cured the surge but it still stumbled a little at idle, changing the coil cured that (~2000 miles old too).

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Absolutely. I appreciate any help I can get!

1-When I went back to original PROM is when I got my miss at 2k. With tuner PROM I had the horrible idle, honestly in the last few miles there I really couldn’t comment on drivability as idle was so bad I barely made it up my street. But also keep in mind tuner prom had been in truck working for 4+ years

2- distributor is a DUI from summit. Previously our OEM dizzy went bad. I replaced with a jegs brand in 2018. 2 weeks ago battling this issue I took off cap and saw cracks in pickup. Replaced entire dizzy thinking that was my problem.

3- tach jumping appears to be random at this point. I can’t say for certain exactly when it does it. I feel like it’s bounced around near 2k but also on fast idle around 1k.

Coil was new a few years ago (maybe 5). I also have 2 spare ICMs I could try

The DUI dizzy did say it should be used with a 45,000 volt coil, which according to what I had looked at is what I have.

I testing coil with multimeter and it passes according to what I read online. However if signs point to that I’m not opposed to changing it. Truck is driving me nuts at this point

Tiger joe said:

i know the truck was running rich with old prom because it burned your eyes, choked you out in the garage with the raw fuel smell.

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The catalyst(s) are destroyed.

Tiger joe said:

also i was always taught -backfire out intake- lean, backfire out exhaust- rich. so im using that logic that when i get this horrible miss driving down the road, and hear a backfire out the tailpipe, its rich.

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I suppose that's true in general. There will be exceptions--burnt intake valves can cause intake backfires, for example.

Tiger joe said:

when you say- spark tester calibrated for HEI- what exactly do you mean? one of the ones where you can adjust the gap to see if spark will jump X gap? i have one of those somewhere i need to dig out if so

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That would work. Not my favorite style, but sufficient for the diagnosis.

The point is, the series of ohmmeter tests can definitely point out a defective coil. The ohmmeter testing CANNOT confirm a "good" coil. The coil can pass the ohmmeter testing and still be defective. Thus the need to verify the voltage output which also verifies the insulation quality of both the case and the insulation on the primary and secondary wires inside the coil.

I prefer this style spark-tester:
www.amazon.com/dp/B003WZXAWK/?coliid=I3S98D7T1J0RLJ&colid=2VLYZKC3HBBDO&psc=1&ref_=list_c_wl_lv_ov_lig_dp_it

But what you have is OK.

Tiger joe said:

keep in mind tuner prom had been in truck working for 4+ years

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Which leads me to believe that there's something beyond the PROM that's gone bad.

Tiger joe said:

3- tach jumping appears to be random at this point. I can’t say for certain exactly when it does it. I feel like it’s bounced around near 2k but also on fast idle around 1k.

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Most-likely an ignition-related failure causing the tach problem.

Tiger joe said:

Coil was new a few years ago (maybe 5). I also have 2 spare ICMs I could try

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Are they known-good, or are they questionable?

Tiger joe said:

I testing coil with multimeter and it passes according to what I read online. However if signs point to that I’m not opposed to changing it.

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List the ohmmeter/multimeter readings. Describe how you're testing.

But as said...you need to verify spark power, also.

The 94-95 year 454 TBI is where fuel pressure went to 26-32 psi.